Downhole steam generators have been found to be an expedient means for injecting large quantities of high quality steam directly from the base of oil drilling wells for the purpose of stimulating petroleum production. Indirectly fired steam generators such as is shown in U.S. Pat. No. 4,243,098 to Meeks et al form steam within a heat exchanger situated in heat transfer relationship to a downhole combustor whereupon the system ejects the steam into the intended petroleum formation while redirecting the exhaust gases of the combustor to pass upwardly along a flue for discharge into the atmosphere at the well head. In contrast, direct fired steam generators such as the one described in U.S. Pat. No. 4,336,839 to Wagner et al inject a mixture of exhast gases and steam from a downhole combustor directly into the underground formation.
The indirect downhole steam generator has many advantages over the direct type, one being that the combustion chamber of the indirect type can be operated at much lower pressures because the indirect-type keeps the exhaust gases of the combustor separate from the steam. As a result, the pumps for delivering the fuel and oxidizer to the indirect downhole steam generator are smaller and less complicated than those of the direct type.
However, the combustor exhaust gases of the indirect downhole steam generator most often comprise significant amounts of nitrogen oxides, sulfur oxides and related acids and acid anhydrides. These highly corrosive and toxic substances chemically attack the walls of the exhaust flue and present significant problems in regard to air pollution. The corrosion in the well casing becomes especially acute at and beyond the point along the length of the well casing where the exhaust gases become sufficiently cooled to allow water vapors and acid vapors contained therein to condense. This water becomes highly acidic and clings to the interior walls of the well casing and to anything contained therein.
In regard to the problem of air pollution, the concentrations of nitrogen oxides and sulfur oxides in the exhaust of the indirect systems often far exceed allowable environment standards. This problem is especially acute when the combustor is fired with low cost, high-sulfur content fuels.
U.S. Pat. No. 3,918,521 to Snavely, Jr. et al describes a system for cleaning sulfur oxides from the flue gases of an above-hole steam generator wherein the flue gases of the combustor and a flow of treated alkaline water are directed in counterflow directions within an above-hole emission scrubber vessel, the water being then collected and directed to a settling tank for the removal of the calcium sulfite contained therein. The system pretreats the alkaline water with an oxidation-inhibitor to prevent the oxidation of the calcium sulfite to calcium sulfate. As is evident from the disclosure, these types of systems requires the emplacement of a substantial number of different types of complicated machinery at every well head, which requirement makes such systems costly both to acquire and to operate. Consequently they are economically unfeasible and unsuitable for wide-spread use.